[Diverted]

Ted
It should be easy to check if it's been rewired by checking where the filament pins are wired. 
According to my book filament pins are:
5U4 has them on pins 2-8
5X4 has them on pins 7-8
The wrong tube shouldn't hurt anything here it just won't light up.
Just check with the tube out (no power), there should be pretty low ohms readings when you find the right ones. Remember these are only 5volt windings. But watch out if you power it up and check voltages there will be very high voltage too!

Thanks Chris! I will wait a while before I start digging in; replacing out electrolytics at the very very least, and fixing some obvious problems (like a cracked resistor I found).
It's very very encouraging to know though that the wrong tube shouldn't cause any larger issues. To me that means that either (1) the last person swapped that in, didn't get anything and stored the radio or (2) the socket's been rewired. If that's the case I would like to wire it back to the appropriate 5X4.

One question: How do I access the underside of the rectifier socket? I've never dealt with a radio that had the rectifier incorporated into the transformer shell.

Thanks!